The analog circuit of FIG. 1 represents one prior art solution to the problem of sensing one of a plurality of input tone frequencies in the presence of noise. It may be referred to as a maximum likelyhood quadrature tone detector for an unknown input phase. This prior art analog device suffers from lack of flexibility in that it must be designed for a single frequency and a predetermined bandwidth. It is also difficult to integrate a circuit of this type with other circuits of a using system.
Another prior art system which provides digital detection of incoming tones is described by A. D. Proudfoot, "Simple Multifrequency Tone Detector", Electronics Letters, Oct. 19, 1972, Vol. 8, No. 21, pp. 524-525. This system does not identify the phase of the incoming signal and it requires a complex digital reference signal for each incoming signal to be identified.